1. Field of the Invention
The present invention pertains to speed control mechanisms for miniature engines, and more particularly to an exhaust throttle for a miniature thermal engine, such as are used on radio-controlled model aircraft and cars.
2. Description of the Related Art
Miniature thermal engines have been developed for use in propelling model craft, such as aircraft, boats, and wheeled vehicles. Remote control of these model craft is achieved by hand-held radio transmitters used in conjunction with receivers and servos mounted in the model craft. This provides for control of a number of functions, including speed and directional control.
Various designs have been employed in controlling the speed of miniature engines. The most common method involves an engine throttle controlled by a single servo actuator. In some designs, the throttle is a variable port that changes size to control fuel or air or a mixture of fuel and air flowing into the engine. In another design, the size of an exhaust opening is increased and decreased for respective control of the engine speed.
A typical exhaust throttle design includes a cylindrical sleeve placed over the piston cylinder of the engine. The piston cylinder has an exhaust opening that is sized to allow exhaust to flow out of the piston cylinder at a maximum speed. The throttle sleeve has a corresponding opening formed in its side wall that is rotated into and out of alignment with the exhaust opening. When the throttle sleeve opening is fully aligned with the exhaust port, maximum engine speed (measured in revolutions per minute or RPM) is achieved. Proportional restriction of the exhaust opening results in a corresponding reduction in engine RPM.
Although this prior exhaust throttle design has been adequate for its purpose, it has the disadvantage of unnecessarily restricting the exhaust opening when in the fully aligned position. It also does not provide for adjustment in the size of the opening in the exhaust throttle sleeve. Moreover, very small engines, such as a 0.010 cubic inch displacement single-piston engine using an air bleed carburetor for throttling will have an idle of up to 14,000 rpm in some cases. Heretofore, no control systems have been devised that can achieve an idle speed much lower than the 14,000 rpm range in the 0.010 size of single-piston engines.
The disclosed and claimed embodiments of the invention are directed to a control system for a miniature engine having individual cylinders with at least one exhaust port to exhaust gases from the cylinder that achieves controlled speed range of 6,000 rpm on idle to 30,000 rpm on full speed.
In accordance with one embodiment of the invention, the control system includes at least one sleeve configured to attach to the cylinder and selectively cover and uncover the exhaust port, the sleeve formed of a metal material having a coefficient of thermal expansion greater than the coefficient of thermal expansion of the material of the cylinder to expand and contract in response to increases and decreases in temperature, respectively, of the cylinder and exhaust gases, thereby changing dimensions of a gap between the cylinder and the sleeve to thereby change the rate of blow-by of the exhaust gases through the gap, to thereby control the speed of the engine.
In accordance with another aspect of the invention, the sleeve is formed from two plates or semi-sleeves, each semi-sleeve having a central portion and first and second fastener wings depending from first and second sides thereof, respectively, and an exhaust vent formed in each of the first and second fastener wings.
In accordance with yet another aspect of the invention, first and second doubler plates sized and shaped to match the first and second fastener wings are attached to the first and second fastener wings.
In accordance with a method of the present invention, at least one sleeve is formed of a material having a greater coefficient of thermal expansion than the coefficient of thermal expansion of an engine cylinder; the at least one sleeve is attached to a cylinder having an exhaust port for movement around the cylinder; the sleeve is rotated on the cylinder to uncover the exhaust port for high-speed operation of the engine, and to cover the exhaust port for idle operation wherein the at least one sleeve expands and contracts in response to increases and decreases in the cylinder temperature, respectively, to alter the dimension of a gap between the at least one sleeve and the cylinder to change the amount of blow-by of exhaust gases between a gap, thereby automatically regulating the speed of the engine.